Field of the Invention
The present invention relates to hydrocarbon recovery from a subterranean formation using in-situ combustion.
Background of the Related Art
Heavy hydrocarbons are often too viscous to be produced using only the formation pressure. One method of lowering the viscosity of heavy hydrocarbons in subterranean formations is to flood the formation with steam. Steam increases the temperature of the hydrocarbons in the formation, which lowers their viscosity, allowing the hydrocarbons to drain or be swept towards a producer well. Steam also condenses into water, which can then act as a low viscosity carrier phase for emulsified oil, thereby allowing heavy hydrocarbons to be more easily produced.
An alternative to adding steam generated on the surface is to generate steam and other hot gasses downhole by burning a portion of the heavy oil reserve. This method of recovering heavy oil via in-situ combustion is often referred to as “fire flooding.” One of the more advanced in-situ combustion techniques, known as “toe to heel air injection” (“THAI”), combines horizontal producing wells with vertical air injector wells. The process begins by circulating steam in both wells so that the oil between the wells is heated enough to flow to the lower, horizontal production well. The steam chamber heats and drains oil as the steam fills in the formerly oil-bearing pores between the wells. Steam circulation in the production well is then stopped and air is injected into the vertical injection well only. Oxygen in the air ignites the oil, generating heat and combustion gasses: CO and/or CO2 (carbon oxides), and H2O (steam). A combustion gas chamber now begins to develop outwards from the injection well. As the hot gases permeate the formation, more oil is heated and cracked, reducing its viscosity and allowing it to flow downward along the combustion front boundary into the production well by way of gravity.
In-situ combustion offers a number of advantages in comparison with alternate thermal recovery methods such as “steam flooding” or “steam assisted gravity drainage” (SAGD). For example, in-situ combustion does not require an ongoing external source of fuel to heat water to steam, or an ongoing source of water to heat into steam. This results in dramatic reductions in energy and water treatment costs.
However, the in-situ combustion process is not entirely without drawbacks. For example, as heavy oil or bitumen is heated by gasses ahead of the advancing combustion front, individual droplets or pockets of oil can melt, flow, and fuse into an oil-continuous layer. Initial, incomplete combustion of the oil also forms oxygenates, like alcohols and organic acids, which adsorb onto the formation minerals, oil-wetting them, and thus promoting an oil continuous layer. Hot combustion gases follow the path of least resistance, divert around this oil continuous layer, and heat only the surface of the oil continuous layer that is facing the flame front before exiting through the producing well.
Heating the oil surface that is facing the flame front causes the oil at this surface to fractionate into lighter distilled hydrocarbons that are driven ahead, and heavier residual hydrocarbons that are left in place. For example, in Athabasca oilsand bitumen, in-situ distillation up to 370° C. leaves as much as 70% of the bitumen behind as a viscous tar. At this temperature, the alkyl side chains on the polynuclear aromatic cores of the bitumen molecules crack off and leave behind semi-solid asphalt. The heaviest asphaltic material forms between 525° C. and 565° C. This is an amorphous, sticky material that continues to crack, condense, and dehydrogenate into a hard, glassy, non-porous wall of coke. Iron compounds in the oil can act as powerful catalysts to accelerate this coking.
This wall of coke, once formed, can impede the flow of heat so much that temperatures in excess of 500° C. on the fire side produce less than 27° C. on the production side. Moreover, the wall channels most of the injected air directly into the producing well, slowing the burn rate to a small fraction of that needed to economically advance the fire flood. Unfortunately, there is no known method to dissolve or disperse this wall of coke once formed. It is often necessary to drill a new injector well, either into the coke to burn it off or on the other side of the coke to bypass the wall. However, drilling another well is expensive and does not prevent the formation of a further wall.